Chlorofluorocarbons (i.e., CFCs ) are compounds containing carbon, fluorine and chlorine. Various CFCs have been used for many years as refrigerants, heat transfer media, foam expansion agents, aerosol propellants, solvents and power cycle working fluids. However, there has been recent concern that chlorofluorocarbons such as CCl.sub.2 F.sub.2 (i.e., CFC-12) and CCl.sub.3 F (i.e., CFC-11) might be detrimental to the Earth's ozone layer. Consequently, there is a worldwide effort to find alternative compounds which contain fewer chlorine substituents, preferably compounds containing no chlorine. The industry has focused on the use of perfluorocarbons, tetrafluoroethanes and hydrochlorofluorocarbons in recent years, since these compounds are believed not to contribute significantly to ozone depletion. Of particular interest to the refrigerant industry is 1,1,1,2-tetrafluoroethane (HFC-134a) . This compound can be derived from the intermediates 1,1-difluorotetrachloroethane (CFC-112a), 1,1,1-trifluorotrichloroethane (CFC-113a) and 1,2,2,2-tetrafluorodichloroethane (CFC-114a) . These intermediates are, in turn, the isomers of the more readily available chlorofluoroethanes 1,1,2,2-tetrachloro-1,2-difluoroethane (CFC-112), 1,2,2-trichloro-1,1,2-trifluoroethane (CFC-113) and 1,2-dichloro1,1,2,2-tetrafluoroethane (CFC-114), respectively.
The isomerization of such chlorofluoroethanes by treatment with aluminum trichloride is well documented in the art. For example, U.S. Pat. No. 5,017,732 discloses a process for isomerizing CFC-113 or CFC-114 to CFC-113a or CFC-114a, respectively, by contacting CFC-113 or CFC-114 with an activated aluminum trichloride catalyst. The catalyst is prepared by contacting the chlorofluorocarbon with anhydrous aluminum trichloride in the presence of a selected metal.
Another hydrofluorocarbon of interest is 1,1,1,3,3,3-hexafluoropropane (i.e., CF.sub.3 CH.sub.2 CF.sub.3 or HFC-236fa). HFC-236fa can be prepared by the hydrodechlorination of 2,2-dichlorohexafluoropropane (CFC-216aa) over a rhenium containing catalyst (U.S. Pat. No. 5,068,473). The preparation of CFC-216aa by the rearrangement of 1,2-dichlorohexafluoropropane using fluorinated alumina catalysts has been disclosed (Japanese Patent Application No. 60-78925).
HFC-236fa is useful as a refrigerant, fire extinguishant, heat transfer medium, gaseous dielectric, sterilant carrier, polymerization medium, particulate removal fluid, carrier fluid, buffing abrasive agent, displacement drying agent and power cycle working fluid.
There is interest in developing more efficient methods for obtaining isomerized products of polychlorofluorocarbons containing lesser, preferably, negligible amounts of non-isomerized starting materials. These polychlorofluorocarbons can be used to prepare useful hydrofluorocarbons (HFCs) by hydrodechlorination processes.